1. Preheat the oven to 140°C.
2. Put the lamb in a roasting or baking dish and make incisions on the top of the lamb leg (small cuts) all over the meat with a sharp knife.
3. Stuff the sliced garlic with some of the thyme and rosemary sprigs into the incisions.
4. Mix together the mint, crushed garlic, olive oil, mayonnaise, whole grain mustard, mustard powder, lemon juice and grated rind in a small bowl and rub all over the lamb.
5. Rub the lamb leg with the mix.
6. Put the remaining fresh thyme and rosemary at the base of the roasting tray, and place the leg of lamb on top.
7. Pour the stock and water into the roasting tin around the meat, and cover with tin foil and seal.
8. Roast in the oven at 140°C for 3 1/2 to 4 hours or until tender.
9. Increase the temperature to 180°C remove the tin foil, place back and roast for 30 minutes until golden.
10. Remove from the oven, and transfer lamb to serving board to rest whilst you make the gravy.